Stainless steel

ABSTRACT

A LOW COST FERRITIC STAINLESS STEEL WITH EXCEPTIONALLY GOOD CORROSION RESISTANT PROPERTIES HAVING THE FOLLOWING COMPOSITION:   CARBON 0.03% MAXIMUM. MANGANESE 2.0% MAXIMUM. CHROMIUM 6 TO 12%. ALUMINUM 2 TO 7%. COPPER 0.1 TO 1.5%. MOLYBDENUM 0.2 TO 3.0%. COLUMBIUM 0.2 TO 3.0%. IRON BALANCE.   WHEREIN CHROMIUM PLUS ALUMINUM IS AT LEAST 13.0%, MOLYBDENUM PLUS COLUMBIUM IS AT LEAST 0.8% AND COPPER PLUS MOLYBDENUM PLUS COLUMBIUM IS AT LEAST 1.0%.

United States Patent 3,594,156 STAINLESS STEEL Henry P. Leckie, Griflith, Ind., and Eugene Williams,

Pittsburgh, Pa., assignors to United States Steel Corporation No Drawing. Filed May 29, 1969, Ser. No. 829,110 Int. Cl. C226 37/10 US. Cl. 75-124 2 Claims ABSTRACT OF THE DISCLOSURE A low cost ferritic stainless steel with exceptionally good corrosion resistant properties having the following composition:

Carbon 0.03% maximum. Manganese 2.0% maximum. Chromium 6 to 12%. Aluminum 2 to 7%.

Copper 0.1 to 1.5%. Molybdenum 0.2 to 3.0%. Columbium 0.2 to 3.0%. Iron Balance.

wherein chromium plus aluminum is at least 13.0%, molybdenum plus columbium is at least 0.8% and copper plus molybdenum plus columbium is at least 1.0%.

BACKGROUND OF THE INVENTION This invention relates generally to stainless steel, and more specifically, to a new, low cost, 6 to 12 percent chromium stainless steel having corrosion resistance equal to or better than the conventional 18 percent chromium stainless steels.

The superior corrosion resistance of commercial stainless steels is primarily due to the addition of chrmoium to the alloy in amounts which vary from 4 to 30%. Since the corrosion resistance of stainless steel is more or less a direct function of the chromium content, it is well accepted that those stainless steels having large amounts of chromium are usually superior in corrosion resistance to those having lesser amounts.

The three most common stainless steels in commercial use today are the A181 (American Iron and Steel Institute) Types 410, 430 and 304 which contain about 12, 17 and 18% chromium respectively. Of these three most common stainless steels, AISI Type 304, having the greatest amount of chromium, is the best for conventional corrosion resistance applications.

In addition to being the primary constituent for corrosion resistance, chromium is also the princi al cost. element in commercial stainless steels. Therefore, AISI Type 304 stainless steel, in addition to being superior in corrosion resistance, is also more expensive than AISI Types 410 and 430 or most other lower chromium stainless steel.

Because of the relatively high cost of chromium, there are continuing efforts to find replacements for at least a portion of chromium by other cheaper elements that would reduce production costs yet maintain a reasonably high degree of corrosion resistance.

SUMMARY OF THE INVENTION This invention is predicated upon our development of a new stainless steel wherein a portion of the chromium content is replaced by controlled amounts of aluminum, copper, molybdenum and columbium. This lower cost stainless steel contains chromium in amounts of from 6 to 12 percent, and yet has corrosion resistance characteristics equal to or greater than AISI Type 304 stainless steel.

Accordingly, it is an object of this invention to provide a new low cost stainless having exceptionally good corrosion resistance and containing from 6 to 12 percent chromium and lesser controlled amounts of aluminum, copper, molybdenum, and columbium.

It is another object of this invention to provide a new low cost stainless steel having from 6 to 12 percent chromium and yet having corrosion resistance characteristics equal to or greater than the 18 percent chromium stainless steels.

-It is a further object of this invention to provide a new stainless steel having corrosion resistance characteristics equal to or greater than AISI Type 304 stainless steel and yet utilizing lesser amounts of chromium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Although aluminum, molybdenum, columbium and copper are known to impart some small degree of corrosion resistance to steel, we have found that there are substantial beneficial interactions between certain combinations of these elements which even further contribute to corrosion resistance. Even relatively small combined amounts of chromium and aluminum; molybdenum and columbium; or copper, molybdenum and columbium will render exceptional corrosion resistant characteristics to a steel. We have further found that a combination of all three beneficial interactions can be used as a substitute for a large portion of the chromium in stainless steels without an adverse affect on corrosion resistance. These substituted elements being cheaper in cost, and/or being used in comparatively smaller amounts have the effects of making a high quality stainless steel cheaper to produce and yet maintaining exceptional corrosion resistance characteristics. A stainless steel in accordance with this invention therefore may have corrosion resistance characteristics equal to or better than AISI Type 304 stainless steel while containing about half, or even less than half as much chromium and low cost quantities of aluminum, copper, molybdenum and columbium.

The stainless steel of this invention has the following general composition:

Carbon 0.3% maximum. Manganese 2.0% maximum. Chromium 6 to 12%. Aluminum 2 to 7%.

Copper 0.1 to 1.5%. Molybdenum 0.2 to 3.0%. Columbium 0.2 to 3.0%

The balance of the steel should of course be iron (Le. iron with other usual steel making impurities). Chromium and the chromium substituted elements must be present in the following minimum combined amounts:

Chromium plum aluminum 13% minimum. Molybdenum plus columbium 0.8% minimum. Molybdenum plus columbium plus copper 1.0% minimum.

Although as little as 2% aluminum will be beneficial to the corrosion resistant properties, optimum results are obtained only if aluminum is maintained within a range of 3 to 7% at an ideal chromium content of about 8%. In a like manner, copper, although beneficial at concentrations as low as 0.1%, is ideally maintained at about 1%. Chromium contents towards the higher end of the range will of course provide slightly better corrosion resistance. For most applications however, about 8% chromium, or from 6 to 10% is preferred as a practical good balance between corrosion resistance and economy.

The beneficial eifects of molybdenum and columbium are somewhat proportional to their combined concentration up to a preferred concentration of about 1% each. Increasing the molybdenum and/or columbium content above the preferred 1% and 0.5% respectively will provide only slight improvements in corrosion resistance up to about 3% each. Beyond 3% of either element, no significant improvement is obtained.

Accordingly, the preferred composition of this stainless steel, for optimum corrosion resistance at a minimum cost, would be about 8% chromium, 37% aluminum and 1% each of copper and molybdenum and 0.5% columbium.

The effect of copper on corrosion resistance properties is actually detrimental at chromium contents below 6%,

comparing them with those values for commercially available stainless steels, the passivation tendency, and hence the corrosion resistance characteristics, for any experimental heat can be readily evaluated. A strong tendency to passivate is manifested by an active passivation potential and a low critical current density.

Table I below contrasts the critical current density and chemistry of AISI Type 430 stainless steel and twelve experimental heats, most of which have compositions in accordance with this invention. AISI Type 430 stainless steel is shown to have a critical current density of 7.2 ma./cm. while all experimental heats had critical current densities below that amount, and hence are superior in corrosion resistance.

TABLE I Chemical composition, percent by weight Critical current Mn P S Si Ni Ti N Cr Al 011 Mo Cb ma./cm.

but of course beneficial at chromium levels above 6%. 30 In Table I above, it is readily apparent that the stain- Therefore, the minimum chromium content of 6% is less steels having compositions in accordance with this incrltical to avoid the adverse affects of copper. vention do have substantially lower critical current densi- Nickel and silicon additions were considered for this ties than the A151 Type 430 stainless steel, and hence, alloy, but neither showed any beneficial effect. Nickel is substantially greater corrosion resistance tendencies. Of neither beneficial nor detrimental. Silicon, on the other 35 the twelve experimental heats, only four show critical hand, not only has a detrimental effect on corrosion current densities of less than 2.0 m./cm. lower than AISI resistance, but further adversely affects the metallurgical Type 430, namely Examples 1, 2, 7 and 12. It should be quality of the steel. Therefore, silicon should be kept at noted that these four examples are the only ones not in residual impurity amounts of less than 1.0%. accord with our composition requirements. (Note under- In order to quantitatively evaluate the stainless steels lined figures outside our ranges.) Example 1 has an of this invention, potentiodynamic polarization tests were excessive amount of aluminum; Examples 2 and 7 an performed. This test is based upon the fact that stainless insuflicient amount of copper; and Example 12 an insuffisteels achieve their high resistance to corrosion by their cient amount of molybdenum and columbi um. In addiability to spontaneously passivate (i.e. form a protective tion, Example 2 further characterizes the adverse affect of oxide film) in the corrosive environment. Although active 5 silicon. metal such as iron can be made to passivate electrolyti- Table 11 below contrasts the critical current density and cally by the application of an electrical current or potential chemistry of A181 Type 304 stainless steel and five experiin a proper solution, the stainless metals inherently mental heats according to a somewhat more preferred passivate without the applied force. The driving force for composition of this invention. Table II is substantially the the electrochemical reaction is provided by the currents 50 same as Table I above except that a better stainless steel resulting from local variations in potential at the metal surface. The inherent ability to form a protective film (passivate) requires that this reaction have a thermo- (i.e. AISI Type 304) is used as the standard. Therefore the experimental heats compared thereto are within our preferred composition range.

TABLE II Chemical composition, percent by weight Critical Example current Number 0 Mn P 3 Si Ni Ti N Cr Al Cu M0 Cb nut/cm- AISI 304 2. 8 1 0. 02 0.60 0. 02 0. 01s 6.13 1. 04 1.04 0.53 1.2 0. 53 0. 005 0. 000 5.18 1.10 1. 04 0. 53 2. 4 0. 50 0. 005 0.003 5. 06 1. 06 1. 05 0. 1. 0 0. 50 0. 006 0.004 4. 80 1. 07 1. 05 0. 50 2. 5 0. 51 0. 005 0. 000 4.13 1. 07 0. 66 0. 40 2. 5 0. 53 0. 005 0. 000 4. l8 1. 00 0. 68 0. 41 1. 5 0. 52 0. 005 0. 006 4.18 1. 06 0. 07 0. 41 2. 0 0. 54 0. 006 0. 008 6. 13 1. 04 0. 53 0. 48 1. 7

dynamic and k1net1c tendency greater than any other From the two tables above it 1s apparent that AISI Type electrode reaction that might possibly take place.

A metal is passive if, on increasing the electrode potential towards more noble values the rate of anodic dissolution in a given environment under steady-state conditions becomes less than the rate at same less noble potential. The tendency for passivation of stainless steels is a function of both a critical current density and a critical potential. Therefore, by measuring these parameters and 304 stainless steel is indeed superior in corrosion resistance to AISI Type 430. Type 304 has a critical current 70 value of 2.8 ma./cm. whereas Type 430 has a critical current value of 7.2 ma./cm. This variation is of course well accepted.

In Table II above, it is readily apparent that the stainless steels having compositions in accordance with our preferred embodiment of this invention have lower critical current densities than even the AISI Type 304 stainless steel, and hence comparable or better corrosion resistance tendancies.

The above potentiodynarnic polarization tests were verified by exposing samples of our stainless steel along with samples of AISI Type 430 and 304 stainless steels, to the corrosive atmospheres of a coke making plant. After seven months exposure our stainless steels were substantially less corroded than the AISI Type 430 steels and exhibited corrosion resistance comparable to or better than the AISI Type 304 stainless steels.

We claim:

1. A low chromium ferritic stainless steel consisting essentially of up to 0.03% carbon, up to 2.0% manganese, 6 to 12% chromium, 2 to 7% aluminum, 0.1 to 1.5% copper, 0.2 to 3.0% molybdenum, 0.2 to 3.0% columbium and the balance substantially iron, the chromium, aluminum, copper, molybdenum and columbium being present in the following combined minimum amounts:

Percent Chromium plus aluminum 13 Molybdenum plus columbium 0.8 Molybenum plus columbium plus copper 1.0

2. A low chromium stainless steel according to claim 1 having about 8% chromium, 37% aluminum, and about 1% each of copper, molybdenum, and columbium.

US. Cl. X.R.

'75l25, 126C, 126F 

